Catalyst for the production of 2,2-disubstituted propiolactones

ABSTRACT

The present invention relates to a process for the manufacture of 2,2-disubstituted propiolactones from isoanhydrides and formaldehyde, as shown in the following equation: ##STR1## wherein R and R 1  individually may be a straight- or branched-chain alkyl, aryl or aralkyl group having 1 to 10 carbon atoms. The reaction is conducted at a temperature of from about 190° C. to about 400° C. in the presence of the metal oxide-silica gel complex which results from heating the calcined residue of a salt of a metal selected from the group consisting of Ta, Ti, Nb and Zr supported upon a silica gel in the presence of nitrogen and steam at a temperature of from about 650° C. to about 1000° C. which complex has additionally been treated by soaking in a mineral acid at a temperature of from about 25° C. to about 100° C. for from about 1 to about 4 hours.

This is a divisional application of Ser. No. 648,080 filed Jan. 12,1976, now U.S. Pat. No. 4,035,389 which application is in turn acontinuation-in-part of application Ser. No. 609,881 filed Sept. 2,1975, now U.S. Pat. No. 4,024,080 which application is a divisional ofapplication Ser. No. 491,092 filed July 23, 1974, now U.S. Pat. No.3,931,237 which application is a continuation-in-part of applicationSer. No. 394,370 filed Sept. 4, 1973, now abandoned.

The present invention relates to a process for preparing2,2-disubstituted propiolactones by the reaction of an isoanhydride withformaldehyde according to the following formula: ##STR2## wherein R andR¹ individually may be a straight- or branched-chain alkyl, aryl, oraralkyl group having 1 to 10 carbon atoms, preferably 1 to 6 carbonatoms, at a temperature of from about 190° C. to about 400° C.

2,2-Disubstituted propiolactones are useful in the polymer industry as astarting material for synthetic resins and synthetic fibers. They arealso useful in the pharmaceutical industry and have heretofore beenprepared by a variety of methods. For example, in U.S. Pat. No.2,356,459, there is described a well-known method for preparing2,2-disubstituted propiolactones by the addition reaction of dimethylketene and formaldehyde. The known methods for the manufacture of2,2-disubstituted propiolactones, however, can be practiced on acommercial scale only with difficulties and resultant economicdisadvantages.

It is, therefore, an object of my invention to provide a simplifiedmethod for the preparation of 2,2-disubstituted propiolactones.

It is another object to provide a one-step method for the preparation of2,2-disubstituted propiolactones.

Yet another objective is to provide an improved catalyst having longerlife and which catalyst can be readily restored to its initial activity.

Other objects of the invention will become apparent from a considerationof the specification and claims of this application.

The prior literature described a reaction of primarily aromaticaldehydes with anhydrides to give unsaturated acids. These reactions arenormally conducted in the liquid phase using basic catalysts. Aliphaticaldehydes are usually unsuitable for this reaction. In the liquid phase,aldehydes normally react with anhydrides to form gem-diesters. Forexample, formaldehyde, when reacted with butyric anhydride, normallygives methylene dibutyrate (J. F. Walker, "Formaldehyde", 3rd Ed., ACSMonograph Series No. 152, Reinholt, p. 350). No prior literature isknown which describes the condensation of aldehydes with acid anhydridesto produce lactones. U.S. patent application Ser. No. 303,567 filed Nov.3, 1972, discloses a process for producing 2,2-disubstitutedpropiolactones from an isoanhydride and formaldehyde in the presence ofa catalyst consisting of a supported heavy metal oxide. These catalysts,however, are not completely satisfactory because they give substantiallylower yields and conversions to the desired lactone or they loseactivity during use and are difficult to reactivate.

In the process of the instant invention, an isoanhydride having theformula ##STR3## is condensed with formaldehyde to yield a2,2-disubstituted propiolactone having the formula ##STR4## and anorganic acid having the formula ##STR5## wherein R and R¹ individuallymay be a straight- or branched-chain alkyl, aryl or aralkyl group having1 to 10 carbon atoms, preferably 1 to 6 carbon atoms. The reaction iscatalyzed by the acid treated metal-oxide silica gel complex whichresults from heating the calcined residue of silica gel and awater-soluble salt of a metal selected from the group consisting oftantalum, titanium, niobium and zirconium in the presence of nitrogenand steam at a temperature of from about 650° C. to about 1000° C. andpreferably less than about 900° C. which complex has been additionallytreated by soaking in a mineral acid such as HCl, HF, HBr, HNO₃, H₂ SO₄,or H₃ PO₄.

Good results are obtained when the calcined residue is heated at atemperature of from about 730° C. to about 780° C. for a period of fromabout 3 to about 6 hours. Following this steam treatment the catalyst iscooled and soaked in dilute HCl, approximately 5%, for about 2 hoursusing temperatures of from about 25° C. to about 100° C.

These catalysts give almost complete reaction of formaldehyde withisobutyric anhydride to form pivalolactone and, to a minor degree, asecondary reaction product, isobutyroxypivalic anhydride. The by-productappears to be formed from the secondary reaction of pivalolactone withisobutyric acid to form isobutyroxypivalic acid, which then interchangeswith excess isobutyric anhydride as shown below. ##STR6##

The by-product isobutyroxypivalic anhydride (hereinafter designated asIBPVA) is useful since it may be converted to pivalolactone by aseparate step. Formaldehyde in the instant reaction may not be readilyrecovered. It is therefore desirable that conversion of formaldehyde tothe useful products pivalolactone and/or IBPVA be as high as possible.

A common method of catalyst regeneration is to burn carbonaceousmaterial from the catalyst at temperatures of up to about 550° C. The550° C. temperature is selected since above about 600° C. silica gelbegins to sinter and lose its structural properties. Since it had beendetermined that heating at 550° C. had very little effect on thecatalyst activity of a heavy metal oxide catalyst, it was quitesurprising that heating a heavy metal oxide-silica gel catalyst totemperatures of from about 650° C. to about 1000° C. in the presence ofsteam produced a highly selective catalyst with long life which could bereadily regenerated.

Liekwise, it was quite surprising that certain acid treatments could beused to impart additional catalyst activity and/or selectivity. Onewould normally predict that mineral acids absorbed on a support such assilica gel would rapidly lose activity because of the tendency to leachthe acid from the support. In the instant invention the acid apparentlyinteracts with the metal oxide and/or silica gel to form a stablespecies since increased catalyst life is one of the results of the acidtreatment.

The supported metal oxide catalysts of the instant invention areconveniently formed by mixing one of their water-soluble salts such as anitrate, acetate, oxalate, or ammonium salt with a silica gel and thenremoving the water by evaporation. Calcining the material in nitrogen atfrom about 400° C. to about 600° C. and then in air at from about 400°C. to about 600° C. produces the desired silica gel supported metaloxide. A preferred temperature range for the calcining step is fromabout 500° C. to about 550° C. If desired, the metal oxide can beprecipitated directly upon the support by use of a suitable chemicalreaction. A relatively low surface area (340 square meters per gram) andlarge pore volume (1.15 cc. per gram) silica gel has been found to beparticularly effective.

The silica gel supported calcined metal oxide is then heated in anitrogen steam mixture at a temperature of from about 650° C. to about1000° C. until the desired metal oxide silica gel complex is formed,usually from about 2 to about 10 hours. The nitrogen is used tofacilitate more uniform heat distribution. Good results have beenobtained with N₂ to H₂ O ratios of from about 0.1:1 to about 10:1.

During the heat treating cycle the relationship of time of treatment totemperature may be varied considerably. Higher temperatures requireshorter treatment times and vice versa. An excellent catalyst has beenobtained by steam treatment in nitrogen at 760°-780° C. for 4 to 6hours. It is essential, however, that the steam treatment be in the650-1000° C. range. A more practical measurement to obtain a catalyst ofgreater activity, selectivity and life span is based on the volume ofthe solid catalyst. When the catalyst being treated at 650°-1000° C. hasbeen reduced in volume by not less than 5 percent and not more than 20percent, the desired catalyst complex has been attained.

The importance of treatment at 650°-1000° C. is further described asfollows. If insufficient heat treatment is used, the catalyst is tooactive and produces considerable decomposition resulting in lowconversions and yields to lactones. On the other hand, if too much heatis applied the catalyst begins to lose activity which results in lowconversions and a short lifetime.

Following the aforementioned heat treating, the catalyst is allowed tocool and is then soaked in a mineral acid. Mineral acids which have beenfound useful include HCl, HF, HBr, HNO₃, H₂ SO₄ and H₃ PO₄. Thetemperature used and the time of soaking depend primarily on the acidstrength and concentration. The effectiveness of the treatment alsodepends on the type and strength of the acid. For example, HCl proved tobe very effective whereas phosphoric acid gave only a small degree ofimprovement in catalyst efficiency. Other acids fall in between thesetwo extremes.

An exact description of the catalyst complex is not available. It isbest characterized by the unique, greatly improved properties itexhibits when compared to analogous catalyst or those prepared byalternate methods.

In a preferred embodiment of the subject invention an aqueous solutionof tantalum oxalate is used as a convenient source of soluble tantalumfor the deposition of tantalum oxide on the silica gel. The catalyst isprepared by soaking the silica gel in an aqueous solution of thetantalum oxalate, removing the water by evaporation, calcining the solidresidue in nitrogen at a temperature of about 550° C. for 11/2 hours andthen in air at a temperature of about 550° C. for 11/2 hours, andheating the resultant silica gel-tantalum oxide mix in a nitrogen steammixture at a temperature of from about 650° C. to about 1000° C. for 2to 8 hours. The heat treated catalyst is then cooled and soaked in asolution of dilute HCl (approximately 5%) for about 2 hours at atemperature of between about 25° C. and about 100° C. A lowertemperature is normally preferred since higher temperatures can produceexcessive acidification and somewhat reduce catalyst selectivity. Ahigher temperature however will normally produce a slight increase incatalyst life.

Acid treatment of freshly prepared metal oxide silica gel catalyst asdescribed above greatly increases the effective life of the catalyst.The overall catalyst activity is also increased. This is shown by anincrease in formaldehyde conversion to the disubstituted propiolactoneand products derived from the disubstituted propiolactone.

Equally important, the acid treatment provides a very good method forreactivating the catalyst after it has lost activity from repeated use.Simple burning of the catalyst to a clean state with air, as is normallydone, cannot completely restore the catalyst activity. Burning thecatalyst clean in the presence of steam at 550° C. produes a significantimprovement in regeneration although after the second regenerationhigher concentrations of unreacted formaldehyde are noted in thereaction effluent. Acid treatment as described herein restores theactivity of the catalyst. The acid treatment for regeneration is mosteffectively carried out by burning the catalyst clean of anycarbonaceous deposits, cooling it and soaking it in a dilute aqueoussolution of the desired mineral acid. Temperatures of from ambient toslightly less than 100° can be used. As in the treatment of freshcatalyst, the most desirable temperature will depend on acid type,strength and time of treatment.

Optimum process conditions such as contact time, temperature, amount ofdiluent gas and feed composition will vary for the different acidtreated metal oxide-silica gel complex catalysts. In general, the bestresults are obtained at a contact time of from about 0.5 to about 2.5seconds, although this may vary over a much broader range, such as fromabout 0.1 second to about 5.0 seconds.

Preferably the temperature selected will be sufficient to insurevaporization of the reactants and the products. The process may beoperated at temperatures of from about 190° C. to about 400° C. Apreferred temperature range is from about 240° C. to about 350° C.

Suitable anhydrides include isobutyric, 2-ethylhexanoic, 2-phenylpropionic, 2-ethylpropionic, 2-ethylbutyric, and2-methylpentanoic.

Formaldehyde may be fed as a gaseous monomer, as a trioxane solution, oras a paraformaldehyde slurry. It has been found that the formaldehydeconversion to lactone is dependent on the amount of anhydride fed. Amolar ratio of from about 1.15:1 to about 5:1, preferably from about 3:1to about 4:1, of anhydride to formaldehyde (as trioxane) in the feedmixture produces good results. The optimum ratio will depend uponvarious manufacturing considerations, such as refining and recyclng ofunreacted feed materials. There appears to be no upper limit to thisratio other than practical manufacturing considerations which arise whena large excess of one material is introduced into a system. In general,a higher anhydride to formaldehyde ratio gives higher formaldehdyeconversion, but also decreases the percentage of lactone in the product.

The reaction may be carried out at atmospheric, subatmospheric, orsuperatmospheric pressure. If desired, an inert diluent gas may beutilized to facilitate feeding of the reactants, control of contacttime, etc. Good results are obtained at atmospheric pressure using aninert diluent gas, usually in a molar ratio of gas to organic feed offrom about 1:10 to about 20:1, preferably about 1:1 to 6:1, and mostpreferably from about 2:1 to 4:1. A suitable inert diluent gas is anygas, such as N₂, argon, helium, gaseous hydrocarbons and compounds whichare readily vaporized such as benzene, which does not react with eitherthe reactants or the products under the conditions of the reaction.

The process of the invention is illustrated in greater detail by thefollowing examples, which are all conducted at atmospheric pressure, butit will be understood that these examples are not intended to limit theinvention in any way, and obvious modifications will occur to thoseskilled in the art.

EXAMPLE 1

This example illustrates the effectiveness of an acid treatment forimparting additional catalyst life by reactivating a used catalyst.

To a 600 milliliter beaker are charged 60 milliliters of tantalumoxalate solution (10.32 grams as tantalum or 12.6 grams as tantalumoxide) and 240 milliliters water. To this are added 100 grams DavisonG-59, 7-10 mesh silica gel and the mixture is left standing overnight.The mixture is transferred to a large evaporating dish and taken todryness on a steam bath. A small amount of powder is removed bycollecting the cataylst on a 20 mesh screen.

To a conventional Vycor reactor (30 millimeters by 2 feet) are charged36 milliliters of Vycor chips, 150 milliliters (99 grams) of the abovecatalyst and 100 milliliters of Vycor chips for preheat. The reactor isbrought to 550° C. and nitrogen is fed at 8.18 moles per hour forapproximately 1.5 hours. Air at 8.18 moles per hour is then substitutedfor the nitrogen for an additional 1.5 hours. At this time the air isshut off and nitrogen at 8.18 moles per hour is again started. Waterfeeding is started through a preheater (approximately 100° C.) at a rateof about 180 milliliters per hour and the temperature is rapidly broughtto 760°-780° C. and held for six hours. After the heat is shut off,steaming is continued until the reactor temperature decreases to about550° C. at which point water feeding is discontinued. After cooling, thecatalyst is removed from the reactor and has lost 14 percent of itsoriginal volume to a final volume of 129 milliliters (60.4 grams).

The following procedure is followed for the generation by acid treatmentof a deactivated catalyst.

The deactivated catalyst (50 milliliters) is burned clean by feeding N₂at 3.5 moles per hour and air at 0.5 mole per hour while feeding waterthrough a preheater at approximately 60 milliliters per hour. Thetemperature is increased to 550° C. and these conditions are held for15-30 minutes. At this point the N₂ is decreased to 3 moles per hour andair increased to 1 mole per hour for approximately 30 minutes. Then 2moles per hour of each is fed for 30 minutes and finally only air (4moles per hour) and steam are fed for 1.5 hours. The catalyst is cleanof carbon but has a slight yellow discoloration while hot. It is almostwhite when cooled in N₂ .

The water and air feed are stopped and the catalyst is cooled in N₂ to25°-35° C. After stoppering the bottom of the reactor, it is filled(appproximately 130 milliliters) with 5.5 percent HCl (25 millilitersconcentrated HCl and 175 milliliters H₂ O) and allowed to stand atambient temperature for two hours. The HCl is drained and the catalystwashed portionwise with a total of 400 milliliters of water. The reactoris put back under N₂ and dried by heating to reaction temperature andthen put in service at the usual operating conditions.

The general procedure followed for testing all catalysts is describedhere. The reactor used is a 22 millimeter by 2 foot Vycor tube heatedwith a three-element furnace and charged with 35 milliliters Vycorchips, 50 milliliters catalyst and 90 milliliters Vycor chips forpreheat. A nitrogen purge of 1.25 moles per hour is used and thereactant feed rate is held at 60-61 milliliters per hour. The operatingtemperature is controlled at 256°-266° C. and a 3 to 1 molar ratio ofisobutyric anhydride to formaldehyde (fed as trioxane) is used as feed.Gas liquid chromatograph (4 foot 20 M TPA Carbowax column) is used toanalyze the reaction product. A composite sample for each run isstripped at approximately 140° C. at 1-2 millimeters to determinepercent high boilers which are used to correct chromatographic results.

Following this procedure 50 milliliters of the fresh catalyst withoutacid treatment are tested in continuous operation. Each run is made fora varying number of days and the results obtained with fresh catalyst,with catalyst after air and steam regeneration, and with catalyst afterair and steam regeneration plus acid regeneration are summarized inTable I.

Total formaldehyde conversion to pivalolactone (PVL) andisobutyroxypivalic anhydride (IBPVA) is a direct measure of the catalystactivity and life. As shown in Table I fresh catalyst (Run 1) andcatalyst burned clean with air and steam (Run 2) run only three dayswith a total formaldehyde conversion to PVL and IBPVA of 80 percent orbetter. The same catalyst burned clean with steam and air and then acidregenerated (Run 3) as described above runs eight days at an 80 percentor better total formaldehyde conversion. A similar increase in catalystlife is also illustrated by formaldehyde conversion to pivalolacetone.Runs 1 and 2 run for three and four days, respectively, at a 70 percentor better conversion. Formaldehyde conversion to PVL is still 71 percentafter 10 days using the acid regenerated catalyst as shown in Run 3.

Isobutyric anhydride yields to pivalolactone and isobutyroxypivalicanhydride are similar in all three runs. This illustrates that acidtreatment of the catalyst (Run 3) is not detrimental to catalystselectivity.

Run 3 in Table I clearly demonstrates the effectiveness of acidtreatment for reactivating the catalyst and for imparting a substantialincrease in the length of time the catalyst retains a high activity.After Run 3 the catalyst activity and life may be restored again byrepeating the procedures used for Run 3.

EXAMPLE 2

This example illustrates the effectiveness of an HCl treatment forimproving the useful lifetime of a freshly prepared catalyst.

A 500 milliliter portion of a nominal 10 percent tantalum (as the oxide)on Davison G-59 silica gel which has been calcined at 550° C. withnitrogen, air and the nitrogen again is charged to a Vycor tubularreactor and heated to 750° ± 10° C. in the presence of a flow of air at500 milliliters per minute and water at 300 milliliters per hour for 2hours. After cooling, the recovered catalyst exhibits a 5 percent lossin volume. This is referred to as the steam treated catalyst.

A 50 milliliter portion of this steam treated catalyst contained in atubular reactor is heated to 250°-265° C. in a nitrogen flow of 465 ccper minute and a solution of trioxane in isobutyric anhydride (1:3 on amolar basis) is vaporized and passed over the catalyst at a rate ofabout 57 milliliters of liquid feed per hour. The results of the run aresummarized in Table II (Run 1 ).

A 70 milliliter portion of the fresh steam treated catalyst is coveredwith 140 milliliters of 1.5 N aqueous hydrochloric acid and held atambient temperatures for two hours. The excess liquid is decanted fromthe solid residue, and the solid granules are washed by decantation withfour 140 milliliter portions of water and finally dried on a steam bath.

A 50 milliliter portion of this acid treated catalyst is placed in thepreviously described tubular reactor and heated to 250°-265° C. in anitrogen flow of 465 cc. per minute. A solution of trioxane inisobutyric anhydride (1:3 on a molar basis) is vaporized and passed overthe catalyst at a rate of about 64 milliliters of liquid feed per hour.The results of this run are also summarized in Table II (Run 2).

It can be seen that the effective lifetime of the catalyst is extendedas indicated by the total formaldehyde conversion.

The hours of operation wherein a formaldehyde conversion of 79 percentor better are extended to 79 (Run 2). This compares to 59 hours ofoperation to the 79 percent formaldehyde conversion without acidtreatment (Run 1).

The selectivity of pivalolactone remains essentially unchanged.

EXAMPLE 3

This example illustrates the ineffectiveness of treating the silica gelsupport alone with hydrochloric acid. The reactor as described inExample 1 is charged with 50 milliliters Davison G-59 silica gel,stoppered and filled with a mixture of 175 milliliters water and 25milliliters concentrated hydrochloric acid. This is heated at 80°-95° C.for two hours. The reactor is then drained and the silica gel washedportionwise with 400 milliliters of distilled water. The catalyst isthen dried by heating to 100°-150° C. for 0.5 to 1.0 hour with anitrogen purge.

A typical run is then made at 255°-265° C. feeding a 3:1 mixture ofisobutyric anhydride to trioxane.

The pivalolactone concentration falls from an initial value of 4.6 to2.6 percent after 5 hours. For the 5 hour period formaldehyde conversionto pivalolactone is 16 percent and the isobutyric anhydride yield topivalolactone is 47 percent.

EXAMPLE 4

This example illustrates the effectiveness of treatments with phosphoricacid, both at ambient temperatures and when warmed, in promoting theactivity of the catalyst and in extending its effective lifetime.

A 45 milliliter portion of a steam treated catalyst contained in atypical tubular reactor is heated to 250°-265° C. in a nitrogen flow of200 milliliters per minute and a solution of trioxane in isobutyricanhydride (1:3 on a molar basis) is vaporized and passed over a typicalsteam-treated tantalum oxide-silica gel catalyst at a rate of about 58milliliters of liquid feed per hour. The results of the run aresummarized in Table III (Run 1). After the run is completed, thecatalyst is burned clean at 550° C. by gradually replacing a nitrogenstream with air and steam. Full air and steam are passed through the hotbed (550° C.) until the carbon deposits are removed and the catalyst bedappears clean visually. The catalyst is then cooled to ambienttemperature under nitrogen.

The cleaned catalyst is held in contact with 1.5 N phosphoric acid fortwo hours at ambient temperature, washed with four 100 milliliterportions of water and dried at 200° C. in air.

When a feed the same as in the previous run is fed over the catalyst at255°-265° C. at a rate of about 65 milliliters of liquid feed per hourand with a nitrogen flow of 200 milliliters per minute, the resultsshown in Table III (Run 2) are obtained.

After a second steam-air cleaning, the catalyst is treated with 1.5 Nphosphoric acid at 80°-100° C. for two hours. After washing and drying,the catalyst is heated to 250°-265° C. and the vaporized feed is fed atthe rate of about 61 milliliters of liquid feed per hour with a nitrogenflow of 465 milliliters per minute. The results are summarized in TableIII (Run 3).

As shown in Table III, total formaldehyde conversion to PVL and IBPVAdrops to 51 percent after 23 hours using untreated catalyst (Run 1) butis still 62 percent after 28 hours when ambient temperature acidtreatment with phosphoric acid is used (Run 2). when heat is used duringthe phosphoric acid treatment (Run 3), the total formaldehyde conversionis still 57 percent after 70 hours.

EXAMPLE 5

This example illustrates the effectiveness of a treatment with aqueousnitric acid in promoting the lifetime of the catalyst.

A 50 milliliter portion of a steam treated tantalum oxide-silica gelcatalyst contained in a tubular reactor is heated to 250°-265° C. in anitrogen flow of 465 milliliters per minute and a solution of trioxaneis isobutyric anhydride (1:3 on a molar basis) is vaporized and passedover the catalyst at a rate of about 65 milliliters of liquid feed perhour. The results of the run are summarized in Table IV (Run 1).

A second 50 milliliter portion of the same steam treated catalyst whichhas been similarly used in pivalolactone production is burned clean in asteam atmosphere at 550° C. by gradually replacing a nitrogen streamwith air. Full steam and air are passed through the hot bed (550° C.)until the carbon deposits are removed and the catalyst appears cleanvisually. The catalyst is then cooled to ambient temperature undernitrogen.

The cleaned catalyst is then held in contact with 5 percent aqueousnitric acid for 2 hours at ambient temperature, washed with four 200milliliter portions of water, and dried at 260° C. in nitrogen.

The same feed composition as used in Run 1 is vaporized and fed over thecatalyst at 250°-270° C. at a rate of about 62 milliliters of liquidfeed per hour and with a nitrogen flow of 465 milliliters per minute.The results are shown in Table IV (Run 2).

The catalyst is burned clean as in Run 2 and contacted with 10 percentaqueous nitric acid for 2 hours at 50°-60° C., washed with four 200milliliter portions of water, and dried at 260° C. in nitrogen.

The same feed composition as in Run 1 is vaporized and fed over thecatalyst at 250°-265° C. at a rate of about 60 milliliters of liquidfeed per hour and with a nitrogen flow of 465 milliliters per minute.The results are shown in Table IV (Run 3).

Total formaldehyde conversion to PVL and IBPVA is 73 percent after threedays using the catalyst with no acid treatment (Run 1). Treating thecatalyst with nitric acid as described gave 81 percent (Run 2) or 84percent (Run 3) total formaldehyde conversion after three days. Ineither case, the isobutyric anhydride yield remained relativelyunaffected at about 90 percent.

                  TABLE I                                                         ______________________________________                                        Days                                                                          of       CH.sub.2 O Conversion to                                                                      IBA Yield to                                         Run  Opera-  PVL    IBPVA  Total PVL  IBPVA  Total                            No.  tion    (%)    (%)    (%)   (%)  (%)    (%)                              ______________________________________                                        1.sup.(1)                                                                          1       65     21     86    58   28     86                               1    2       69     15     84    65   22     87                               1    3       70     11     81    70   17     87                               1    4       68     9      77    70   15     85                               2.sup.(2)                                                                          1       74     12     86    70   17     87                               2    2       76     7      83    76   11     87                               2    3       74     6      80    80    9     89                               2    4       71     6      77    78    9     87                               2    5       69     6      75    79   11     90                               2    6       67     5      72    78    9     87                               2    7       64     5      69    76   10     86                               3.sup.(3)                                                                          1       72     13     85    67   19     86                               3    2       76     10     86    74   15     89                               3    3       77     9      86    77   14     91                               3    4       76     8      84    78   13     91                               3    5       75     8      83    76   12     88                               3    6       75     6      81    80    9     89                               3    7       74     6      80    79   10     89                               3    8       73     7      80    77   11     88                               3    9       73     6      79    79   11     91                               3    10      71     6      77    80   10     90                               ______________________________________                                         .sup.(1) Freshly prepared catalyst as described                               .sup.(2) Catalyst burned clean in air and steam at 550° C.             .sup.(3) Catalyst burned clean in air and steam at 550° C.; soaked     in 5.5 percent HCl at .sup.- 30° C. for 2 hours then washed with       water.     t1 TABLE II-Hours of? CH.sub.2 O Conversion to? -Run No.?          Operation? PVL (%)? IBPVA (%)? Total (%)? -(1)  1 51 34 85 -  7 63 22 85 -     24 67 18 85 - 31 66 17 83 - 59 65 14 79 -(2)  1 47 34 81 -  7 56 27 83 -     29 63 23 86 - 50 63 19 82 - 72 65 15 80 - 79 65 14 79 -

                  TABLE III                                                       ______________________________________                                        Hours of     CH.sub.2 O Conversion to                                         Run No.                                                                              Operation PVL (%)   IBPVA (%)                                                                              Total (%)                                 ______________________________________                                        (1)     1        51        11       62                                                7        54        11       65                                               23        40        11       51                                               47        36         6       42                                               53        30         6       36                                        (2)     1        42        11       53                                                6        52        11       63                                               24        50        11       61                                               28        51        11       62                                        (3)     3        34        28       62                                                7        44        26       70                                               24        50        20       70                                               48        48        12       60                                               70        50         7       57                                        ______________________________________                                         (1)Fresh steam treated catalyst                                               (2)Catalyst after burning clean and treated with 1.5 N phosphoric acid at     ambient temperature.                                                          (3)Catalyst after burning clean and treated with 1.5 N phosphoric acid at     80-100° C.                                                        

                  TABLE IV                                                        ______________________________________                                        Days                                                                          of       CH.sub.2 O Conversion to                                                                      IBA Yield to                                         Run  Opera-  PVL    IBPVA  Total PVL  IBPVA  Total                            No.  tion    (%)    (%)    (%)   (%)  (%)    (%)                              ______________________________________                                        (1)  1       67     16     83    66   24     90                                    2       66     11     77    70   17     87                                    3       63     10     73    74   18     92                               (2)  1       67     15     82    64   22     86                                    2       69     13     82    69   19     88                                    3       70     11     81    72   18     90                                    4       69     11     80    72   17     89                                    5       70     11     81    72   17     89                                    6       68     10     78    70   16     86                                    7       66     10     76    71   17     88                               (3)  1       70     15     85    65   22     87                                    2       73     12     85    74   19     93                                    3       73     11     84    76   17     93                                    4       71     11     82    77   16     93                               ______________________________________                                         (1)Usual steam treated catalyst                                               (2)Catalyst after burning clean and treatment with 5 percent nitric acid.     (3)Catalyst after burning clean and treatment with 10 percent nitric acid                                                                              

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention as described hereinabove and in the appended claims.

I claim:
 1. A catalyst consisting of the acid treated metal oxide-silicagel complex which results from (1) heating the calcined residue of amixture of silica gel and a water soluble salt of a metal selected fromthe group consisting of tantalum, titanium, niobium and zirconium to atemperature of from about 650° C. to about 1000° C. in the presence ofwater vapor and (2) soaking the product of step (1) in a mineral acid.2. The catalyst of claim 1 wherein the catalyst is formed by mixing awater-soluble salt of the selected heavy metal with silica gel, removingthe water by evaporation, and subsequently calcining the material at atemperature of from about 400° C. to about 600° C.
 3. The catalyst ofclaim 2 wherein the calcining takes place at a temperature of from about500° C. to about 550° C.
 4. The catalyst of claim 2 wherein the calcinedresidue is heated, in the presence of water vapor, to a temperature offrom about 730° C. to about 780° C.
 5. The catalyst of claim 4 whereinthe calcined residue is heated, in the presence of water vapor, for aperiod of from about 3 to about 6 hours.
 6. The catalyst of claim 4wherein the calcined residue is heated, in the presence of water vapor,to a temperature of from about 760° C. to about 780° C.
 7. The catalystof claim 6 wherein the calcined residue is heated, in the presence ofwater vapor for a period of from about 4 to about 6 hours.
 8. Thecatalyst of claim 1 wherein the mineral acid is selected from the groupconsisting of HCl, HF, HBr, HNO₃, H₂ SO₄ and H₃ PO₄.
 9. The catalystaccording to claim 1 wherein the mineral acie is utilized as an aqueoussolution thereof.
 10. A catalyst according to claim 1 wherein thecatalyst is soaked in the mineral acid for a period of from about one toabout 4 hours at a temperature of from about 25° C. to about 100° C. 11.A process of restoring activity to a metal oxide-silica gel complexcatalyst, which has lost activity as a result of use, which consists ofthe steps of removing any carbonaceous deposit which may be present andsoaking the said carbon-free catalyst in a mineral acid.
 12. Areactivation process according to claim 11 wherein the carbonaceousdeposits are removed by oxidation.
 13. A reactivation process accordingto claim 11 wherein the catalyst is soaked in a mineral acid selectedfrom the group consisting of HCl, HF, HBr, HNO₃, H₂ SO₄ and H₃ PO₄. 14.The catalyst according to claim 11 wherein the mineral acid is utilizedas an aqueous solution thereof.
 15. A catalyst according to claim 11wherein the catalyst is soaked in the mineral acid for a period of fromabout one to about four hours at a temperature of from about 25° C. toabout 100° C.